What​ ​is​ Apple Activation ​iCloud​ ​Lock and its purpose?

What is iCloud Lock?

Apple introduced the activation iCloud lock on its devices back in 2012, and it is a cloud computing
and storage service that took over from Apple’s MobileMe service.

Simply put, the iCloud enables you to sync up all your Apple devices.
You can link your phone wirelessly to your iCloud account using
your Apple ID as your login credentials.

The iCloud service is available to Apple users freely and runs in the
background without requiring any user-driven actions to keep working.

To access the iCloud service you need to create an iCloud account,
but the iCloud credentials are often similar to that of your Apple ID.
What is iCloud
Link your new or existing iPhone to your iCloud account to
benefit from the full extent of the innovative iCloud features.

iCloud also provides a restore and backup feature that allows you to
store virtually all the content of your phone wirelessly on the cloud.

Some of the other features supported by iCloud include the Back to My
Mac remote access service, Find My Friends, Find My iPhone, and iCloud Keychain.
One of the protective layers of security that came with the iCloud is the Activation Lock.

An iPhone that has been Activation Locked can only be unlocked with the iCloud credentials of its owner. Therefore, buying an iCloud locked phone is equivalent to buying a very expensive paperweight. … It’s a layer of security designed by Apple to protect people from having their phones stolen and then resold by thieves.